Light emitting diode illumination system

ABSTRACT

In various embodiments of the invention, a unique construction for Light Emitting Diodes (LEDs) with at least one luminescent rod and extracting optical elements used to generate a variety of high brightness light sources with different emission spectra. In an embodiment of the invention, forced air cooling is used to cool the luminescent rod. In an embodiment of the invention, totally internal reflected light can be redirected outward and refocused. In another embodiment of the invention, light emitted by the luminescent rod is out-coupled for use in a variety of applications.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.13/968,178, filed Aug. 15, 2013, entitled “LIGHT EMITTING DIODEILLUMINATION SYSTEM” which is a continuation of U.S. patent applicationSer. No. 13/344,815, filed Jan. 6, 2012, (now U.S. Pat. No. 8,525,999,issued Sep. 3, 2012) entitled “LIGHT EMITTING DIODE ILLUMINATIONSYSTEM”, which is a continuation of U.S. patent application Ser. No.12/186,475, filed Aug. 5, 2008, (now U.S. Pat. No. 8,098,375, issuedJan. 17, 2012), entitled “LIGHT EMITTING DIODE ILLUMINATION SYSTEM”,which claims priority to U.S. Provisional Patent Application No.60/954,140, entitled: “LIGHT EMITTING DIODE ILLUMINATION SYSTEM”, filedAug. 6, 2007, which applications are incorporated herein by reference intheir entireties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.12/187,356 entitled “LIGHT EMITTING DIODE ILLUMINATION SYSTEM” filedAug. 6, 2008 by Thomas Brukilacchio and Arlie Connor (now U.S. Pat. No.7,898,665) which application is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

This invention, in general, relates to high brightness illuminationsources and more particularly to the use of light emitting diodes (LEDs)as a source of illumination for medical endoscopy.

BACKGROUND OF THE INVENTION

There is a significant need for high brightness broad band illuminationsources to provide optical fiber coupled illumination for surgicalendoscopy and other applications where extremely high brightness sourcesare needed such as in projection systems and high speed industrialinspection. Prior art typically utilize short arc lamps such as highpressure mercury, metal halide, and xenon. These lamps are capable ofvery high luminous emittance and are therefore suitable sources for theetendue limited fiber optic coupled illumination systems. Approximately85% of the high brightness illumination sources in use in the operatingroom today are based on compact short arc xenon lamps. The problemsassociated with these lamp technologies, however, include poor luminousefficacy thereby requiring high power and associated means of cooling,short lifetime, high voltage operation (typically kilovolts required toturn them on), high cost, and use of mercury which is becoming anenvironmental hazard and is in the process of undergoing regulations innumerous countries throughout the world.

Only recently has there been recognition that LEDs may providesufficient illumination to be used to replace more traditional lightsources in endoscopic illumination systems. In particular, LEDs providemuch improved lifetime, lower cost of ownership, lower power consumption(enabling some battery operated portable devices), decreased coolingrequirements, and freedom form mercury relative to conventional arclamps. Additionally they can be readily modulated which can be asignificant advantage in many applications. To date no LED basedendoscopic illumination system commercially exists that equals orexceeds the luminous intensity of the compact xenon arc lamp systems.The invention described herein has the potential of meeting andexceeding the output of the best arc lamps systems available today

SUMMARY

The invention herein describes a white light illumination system whichincorporates a luminescent rod material which is excited along itslength by a linear array of LEDs. In a preferred embodiment theluminescent material is single crystal or sintered ceramic Ce:YAG(cerium doped yttrium aluminum gamete, Y₃Al₅0₁₂:Ce³⁺) and the LEDs areblue GaN based surface emitting devices. The green and or yellow outputspectrum from the rod is index matched to a collection optic whichconverts the light emitted from the aperture of the rod to a largerdimension with a smaller solid angle to allow it to be imaged to a fiberbundle. The output of the luminescent rod and collection optic could becombined with the output of other directly coupled LED arrays in theblue and green spectral regions to produce white light. While blue andred LED modules can be produced to equal or exceed the brightness ofconventional high brightness light sources such as compact xenon arclamps, the efficiency of LEDs in the true green spectrum in the spectralregion of 555 nm are of comparatively low efficiency and are notsufficiently bright compared to arc lamps.

Typically light generated from LEDs in the spectral region of 555 nm isachieved by applying a thin layer of encapsulant with phosphor suspendedin it directly over LED die emitting blue light. The light from thephosphor particles is partially absorbed and partially scattered. Thecombination of the scattered blue light and the absorbed lightre-emitted as luminescent light at longer wavelengths typically in thegreen and red spectral region produces white light.

The amount of white light produced can be increased by increasing thecurrent density to the LED up to the point where the output of the LEDrolls over and no longer increases with increasing current. Thebrightness of any LED made by in this general configuration isfundamentally limited by the internal and external quantum efficiency ofthe LED die, the quantum efficiency of the luminescent material, theamount of scattering by the particles, the thermal quenching propertiesof the die, and the die junction temperature. In contrast, the presentinvention is not limited by the current density of the LED as the lengthof the rod material could be increased to increase the number ofexcitation LED die and thereby increasing the output. For example, ahigh performance LED die with a 1 mm square area coated with a highperformance phosphor can produce approximately 200 Lumens with a heatsink temperature near room temperature before rolling over and no longerproducing more light with further increases in current density.

A luminescent rod of the present invention with the same 1 mm squarecross sectional area with a length of 50 mm could have on the order of100 LEDs exciting the rod. As will be shown in the detailed descriptionbelow, a conservative efficiency of 30% would result in an output ofmore than an order of magnitude higher photometric power with each LEDoperating at significantly lower current densities. Furthermore, ifhigher output was required the length of the rod could be increasedalong with an increase in the number of LEDs exciting the luminescentrod. Thus the present invention comprises a means of producing output inthe green portion of the spectrum that is of higher brightness than canbe achieved by even the best xenon short arc lamps.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is described with respect to specific embodimentsthereof. Additional features can be appreciated from the Figures inwhich:

FIG. 1 shows a preferred embodiment of the light emitting diodeillumination system. Three spectral coupled sources generally in theblue, green, and red portions of the spectrum are combined to provide ahigh brightness light source capable of providing illuminationsufficient for fiber optic coupled endoscopic illumination medicalendoscopy;

FIG. 2 is a detail of the solid rod luminescense material optical systemcomprised of the Ce:YAG rod, blue LED excitation sources, heat sinks,and index matched output optic. The top view represents a crosssectional view;

FIG. 3 is a preferred embodiment of the combined mirror and Ce:YAGcooling system;

FIG. 4 shows an alternative embodiment containing two luminescent rodsources in series;

FIG. 5 shows various alternative cross sectional shapes;

FIG. 6 shows three different output coupling optics attached to theluminescent material rod and also integrated as part of the rod;

FIG. 7 shows a plot of efficiency of coupling light out of the end ofthe rod assuming different cross sectional shapes (_(——————) circularvs - - - square) as a function of the index of refraction of theattached optic;

FIG. 8 shows the transmission spectrum of a white light source as afunction of wavelength for two thicknesses (1 mm and 50 mm) for a 0.15%doped Ce:YAG rod; and

FIG. 9 shows a spectral plot of the relative intensity versus wavelengthfor three sources (blue, green and red) in the system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to high brightness fiber opticillumination systems. In particular, the present invention represents anLED based light source for improved illumination systems relative to arclamp and other LED based light source systems. The illumination system10 of FIG. 1 is comprised of one or more LED die or die array modules12, 24 and 26 spectrally and spatially combined by means such asdichroic beam splitters 42 and 44 coupled to a common source aperture 52which substantially conserves the etendue or area, solid angle, indexsquared product. A preferred embodiment of the system couples into anoptical fiber bundle to provide the high luminous power and highbrightness required for medical endoscopic applications. Other highbrightness applications include, but are not limited to, projectionsystems, industrial illumination, photo curing, spot lights, and medicalphotodynamic therapy.

Prior to LED based systems conventional arc lamp based projectionsystems were used comprised of a short arc lamp typically of the highpressure mercury, metal halide, or xenon lamp variety. The primarydisadvantage of the short arc technology is lamp life, which istypically in the 500 to 1000 hour range. The cost of the arc lamp itselfand the service cost to replace the lamps over the life of the productcan be many multiples of the original cost of the complete illuminationsystem. This drive hospital costs up which in turn drive the costs ofmedical insurance up. Additional benefits of the LED technology includereduced power consumption, low voltage operation, light intensitystability, ability to control correlated color temperature (CCT) andcolor rendering index (CRI), and the ability to modulate the source. Theability to modulate the source can be a significant benefit. Forexample, most of the endoscopic systems in use today are coupled to avideo camera. Typically video cameras incorporate an electronic shutterand typically the video signal is not integrated continuously. Thus,there is an opportunity to modulate the LED source in synchronizationwith the shutter. During the time when the shutter is closed, the LEDlight source does not need to be on. Thus, for example, if the shutterwas open 50% of the time, the light source could be modulated insynchronization producing 50% less heat. Thus, for the same averageinput power to the LED light source the light output could be increasedby an amount dependant on the operating point of the LED source withrespect to efficiency.

A more conventional approach to producing white light by LEDs is todeposit a phosphor powder, typically of Ce:YAG (cerium doped yttriumaluminum garnet, Y₃Al₅0₁₂:Ce³⁺) suspended in an encapsulant materialsuch as silicone, onto a blue LED die or die array with a peakwavelength between about 445 nm and 475 nm. The light absorbed by thephosphor is converted to yellow light which combines with the scatteredblue light to produce a spectrum that appears white. The apparent colortemperature is a function of the density and thickness of the phosphorsuspended in the encapsulant. While this approach is efficient, theamount of white light produced per unit area per unit solid angle isfundamentally limited by the amount of blue light extracted from theblue LED die or die array, the quantum efficiency of the phosphor, thephosphors thermal quenching, and the back scattering, which is afunction of the particle size of the phosphor or other luminescentmaterial. While it is feasible to place a solid phosphor such as singlecrystal Ce:YAG over the top of the blue LED die or die array, the changein effective path length with angle which increases from normalincidence as the rays approach the plane of the LED die emitting surfaceproduces a change in spectrum with angle resulting in a non-uniform farfield distribution and undesirable color variation. Furthermore, theefficiency of such a device would be limited by the total internalreflection of such a luminescent material due to its high index ofrefraction unless the surface was in contact with an index matchingmedium or included a structure to increase extracted radiance such as aphotonic lattice, surface roughened or micro-lens array.

The heart of the invention of FIG. 1 is the LED source module 12comprised of a central rod 14 of luminescent material such as singlecrystal or sintered ceramic Ce:YAG, and other luminescent materialsincluding:(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z-c)Ga_(z)Si_(c))₅O_(12-c)N:Cea_(a)Pr_(b)with 0<x<1, 0<y<1, 0<z</=0.1, 0<a<=0.2, 0<b<=0.1, and 0<c<1 for exampleLu₃Al₅O₁₂:Ce³⁺, Y₃Al₅O₁₂:Ce³⁺ and Y₃Al_(4.8)Si_(0.2)O_(11.8)N_(0.2):Ce³⁺emitting yellow-green light; and(Sr_(1-x-y)Ba_(x)Ca_(y))_(2-z)Si_(5-a)Al_(a)N_(8-a)O_(a):Eu_(z) ²⁺ where0<=a<5, 0<x<=1, 0<=y<=1, and 0<z<=1 for example Sr₂Si₅N₈:Eu³⁺, emittingred light. Other candidates include(Sr_(1-a-b)Ca_(b)Ba_(c))Si_(x)N_(y)O_(z):Eu_(a) ²⁺ where a=0.002 to0.20, b=0.0 to 0.25, c=0.0 to 0.25, x=1.5 to 2.5, y=1.5 to 2.5, andz=1.5 to 2.5 for example SrSi₂N₂O₂:Eu²⁺;(Sr_(1-u-v-x)Mg_(u)Ca_(v)Ba_(x))(Ga_(2-y-z)Al_(z)In_(z)S₄):Eu²⁺ forexample SrGa₂S₄:Eu²⁺; (Sr_(1-x-y)Ba_(x)Ca_(y))₂SiO₄:Eu²⁺ for exampleSrBaSiO₄:Eu²⁺; (Ca_(1-x)Sr_(x))S:Eu²⁺ where 0<x<=1 for example CaS:Eu²⁺and SrS:Eu²⁺;(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-x)(Al_(1a+b)B)Si_(1−b)N_(3−b)O_(b):RE_(n)where 0<=x<=1, 0<=y<=1, 0<=z<=1, 0<=a<=1, 0<=b<=1 and 0.002<=n<=0.2 andRE is either europium(II) or cerium(Ill) for example CaAlSiN₃:Eu²⁺ orCaAl_(1.04)Si_(0.96)N₃:Ce³⁺; andM_(x)v+Si_(12-(m+n))Al_(m+n)O_(n)N_(16-n), with x=m/v and M comprised ofa metal preferably selected from the group comprising Li, M, Ca, Y, Sc,Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or mixtures includingfor example Ca_(0.75)Si_(8.625)Al_(3.375)N_(0.625):Eu_(0.25) asdisclosed in U.S. patent application Ser. 11/290,299 to Michael R.Krames and Peter J. Schmidt (publication #2007/0126017) which is hereinexplicitly incorporated by reference in its entirety; and nano-phosphorsembedded in a suitable matrix such as high index plastic or glass, withLED die positioned along its length in a linear array of die or a singlelong LED die attached to a high thermal conductivity board 18, such ascopper or aluminum core printed circuit board, which in turn is attachedto heat sink 20. The luminescent rod 14 would have the properties ofhigh absorption of light in one part of the spectrum, blue in the caseof Ce:YAG, emission with high quantum yield in a wavelength regiongenerally longer than the excitation wavelength band, high index ofrefraction to trap a significant portion of the luminescent lightproduced such that it is guided or transmitted down the length of therod toward an emitting aperture 52. The emitting aperture would be indexmatched to an optical concentrator 22 such as a compound parabolicconcentrator (CPC), compound elliptical concentrator (CEC), compoundhyperbolic concentrator (CHC), taper, or faceted optic. Theconcentrators would generally be index matched and of solid dielectric,although liquids could work as well. The purpose of the concentrator istwo-fold. First, it would be made of a material with an index ofrefraction approaching that of the rod (approximately 1.82 for Ce:YAG)and second, it would act to convert the light emitted over a hemisphere(2π steradians) to an area and solid angle that can be readily imagedthrough dichroic beam splitters and re-imaging optics whilesubstantially preserving the etendue (area, solid angle, index squaredproduct) thereby maximizing the brightness.

The output spectrum of the Ce:YAG rod source would cover the rangebetween about 500 nm and 700 nm, with the predominant contribution inthe green spectrum centered around 555 nm. The combination of this lightwith that from a blue LED module 24 would produce white light suitablefor many applications. For medical illumination, however, the relativespectral content is typically required to result in a high colorrendering index (CRI) on the order of 85 or greater. To accomplish thisit is necessary to add additional light in the red spectral region froma third LED source module 26. In FIG. 1 dichroic beam splitter 42 wouldtransmit the red light of LED module 26 and reflect the blue light ofLED module 24. Dichroic beam splitter 44 would transmit the combinedblue and red spectrum of combined LED modules 26 and 24 and reflect thegreen or yellow light of LED module 12. The combined white lightspectrum from LED modules 12, 24, and 26 would then be imaged by lenselements 46 and 50 to stop the input aperture 52 of fiber optic lightbundle 54. The lens elements 46 and 50 could be comprised of multiplelens elements which may include glasses or plastics of differentdispersions to help optimize image quality. The lens systems stop 48would assure that the extent of the far field of the light from each LEDmodule was similar so as not to result in color fringe effects at theedge of the illumination field. The size of each LED source and theircollection optics would be sized such as to produce substantiallysimilar near and far field distributions for each LED module. The lenssystem could also include diffractive or reflective components to helpreduce the number of or optical elements and to reduce overall packagesize. The relative position of the LED modules 12, 24, and 26 areinterchangeable assuming that the dichroic beam splitters were changedin spectral characteristics to accommodate different arrangements. Forexample, LED modules 12 and 24 could be switched in position such thatbeam splitter 42 would transmit red light, reflect blue and green lightand beam splitter 44 would transmit red and green and reflect bluelight. The spectrum of the LED modules in a different system couldinclude ultraviolet through mid infrared light assuming the opticalelements where made of the proper transmitting materials andanti-reflection or reflection coatings. The LED modules 24 and 26 wouldbe comprised of an LED array either index matched or not index matchedto the collection optic depending on the extraction efficiency andmethod of the LED die. For example blue die form CREE (EZ1100) includesa micro lens array such that the benefit from index matching does notcompensate for the increase in the etendue due to the index squaredeffect. Thus for the case of these high performance blue die higherbrightness is achieved by not index matching. The red die that arecommercially available at this time do not typically includemicrostructures on their surface to significantly enhance extractionefficiency and thus do benefit from encapsulation, not from a brightnessstandpoint, but from an efficiency standpoint which due to decreasedthermal load translates into improved performance. The collection opticscould be comprised of similar optics as detailed for the LED module 12,however, in the case of the blue die, the CPC, taper, or otherconcentrator could be designed for no index matching.

Heat sinks 12, 25, and 34 of FIG. 1 could be made out of any highthermal conductivity material including but not limited to copper andaluminum. The LED or LED arrays 16, 30, and 38 would be attached to LEDprinted circuit boards (PCBs) 18, 28, and 36 which would in turn bethermally and mechanically attached to heat sinks 12, 25, and 34respectively. In a preferred embodiment the PCBs would be made out of ahigh thermal conductivity material including but not limited to copper,diamond, aluminum, or composite materials. Ideally the thermalresistance between the back side of the LED die or die arrays would beminimized by direct eutectic attachment, soldering, or thermallyconductive epoxy. The high thermal conductivity PCBs would act as heatspreaders thereby reducing the heat flux into the heat sinks 12, 25, and34. The heat sinks could be cooled by direct convection with air,conduction with various coolant fluids such as water, or radiation intothe surrounding environment. Heat pipes of various constructions havealso been found to work very effectively as heat sinks Heat pipes anddiamond could also be used as the PCB material as they both are veryeffective heat spreaders with performance well above that of purecopper.

FIG. 2 shows a detailed view 60 of the LED module 12 of FIG. 1 from theside and in cross section as indicated in 70. The luminescent rod 14,which in a preferred embodiment would be single crystal or transparentsintered polycrystalline Ce:YAG would be characterized by highabsorption in a spectral region such as blue in the region of 460 nm andvery low extinction for wavelengths greater than the excitationwavelength band above 500 nm to 510 nm. The rod material 14 would alsobe characterized by exhibiting luminescence of the absorbed excitationlight with high quantum yield. Thus the LED array 16 would in apreferred embodiment be comprised of blue LED die such as thosemanufactured by CREE Inc. called EZ1000 which are dimensionally on theorder of 1 mm square by 0.120 mm thick. The light from the LED arraywould be transmitted through the outer wall of luminescent rod 14. Theextinction coefficient of rod 14 would be doped to a level resulting insubstantially all of the blue light being absorbed within the dimensionof the rod prior to exiting the rod through its other side. To theextent that the excitation light was not absorbed with the first passthrough the rod 14, mirrors 72 could be positioned with a reflectivesurface close to the rod so as to cause the excitation light to passback into the rod one or more times to maximize absorption by the rod.The reflectivity of the LED die is on the order of 80% which would alsoact to couple light that was not absorbed on the first pass through therod back into it for another opportunity to be absorbed. The light couldtake multiple passes to be substantially absorbed. Given the finitereflectivity of the mirrors 72 and diffuse reflectivity of the LED die16 it would be best to chose an extinction that would result in theorder of 80% or more of the excitation light being absorbed on the firstpass through the rod 14. Alternatively, the sides of the rod throughwhich the excitation light is not passing initially could be coated witha high reflectivity coating. It would be critical, however, that thereflectivity be very close to 100% so as not to loose substantialluminous power upon multiple reflections as the luminescent light istransmitted toward the output aperture 62. In a preferred embodiment theoutside surface of the rod would not be coated at all so as to allow asubstantial portion of the light generated within the rod to be guidedby total internal reflection (TIR) up the rod toward output aperture 62.The fact that the luminescent material 14 has a relatively high index ofrefraction is fortunate as the higher the index of refraction thegreater percentage of the light that is generated within the rod will beguided by TIR toward the output aperture 62.

The luminescent light generated within the rod 14 would be substantiallyisotropic and thus would travel equally in all directions. Thus half ofthe light that is bound to the rod by TIR would travel in a directionopposite to the output aperture 62 toward mirror 66 which would act tosend the light emitted in that direction back toward output aperture 62,thereby substantially doubling the light reaching output aperture 62.The mirror could also be effectively coated directly onto the end faceof rod 14 in the vicinity of mirror 66.

FIG. 3 shows an alternative embodiment 80 of the mirror elements 66 ofFIG. 2 comprised of modified mirror elements 82 containing the additionof small holes 84 through which high pressure air would cool rod 14 byhigh pressure air impingement. The holes would be sufficiently small asto minimally affect the mirrored surface area of mirrors 82. Highpressure air impingement has several times the film coefficient and thusheat transfer as compared to standard connected low pressure air. Theeffect of the slight increase in the index of refraction of the mediumsurrounding rod 14 on TIR would be minimal. If direct contact coolingfluid was used without the sides of the rod being reflective, the higherthan air index of refraction of the fluid would result in more loss outthrough the sides due to the decreased TIR internal angle, therebyreducing overall LED module efficiency. The reason it may be importantto provide a means of removing heat build up from the rod is that therewould be a small but finite heat absorption, convection and conductionto the rod from the LED array 16 that would cause an increase intemperature of the rod if there were no means of removing this heat.This heat rise would result in reduced LED module performance due tothermal quenching of the luminescent rod material. Increasing thetemperature of the rod material can decrease the quantum efficiency.

FIG. 4 shows an alternative embodiment 120 of LED module 12 of FIG. 1where two modules 12 have been positioned in sequence to form a singlemulti-spectrum source. For example rod 122 of 120 could be made of aluminescent material with properties similar to those described for rod14 for which the excitation band is within the long wavelengthultraviolet spectrum in the region of 240 nm to 420 nm. The hightransmission region of the material would be in wavelengths longer than420 nm and its luminescence could be in the blue to blue-green spectralregion. Likewise rod 124 could have similar absorption properties butcomprise luminescence in the green to red region of the spectrum. Bothrods 122 and 124 would be characterized by high transmission in thespectral region containing wavelengths longer that 420 nm.

The mirror 66 would act to reflect any light transmitted in thedirection opposite output coupler 22 back toward 22. In this way, LEDlight module 120 could contain the full and desired spectrum of thewhite light source and would not require supplemental LED modules 24 and26 of FIG. 1 nor dichroic beam splitters 42 and 44. It would benecessary to use an index matching material between the two rods 122 and124 such as melted Schott SF6 glass or other suitable index matchingmaterial. Alternatively, a single material or ceramic such as YAG(yttrium aluminum garnet) could use different dopants in the regionscorresponding to rods 122 and 124 such that the rod is continuous andthere is no need for an index matching medium. Alternatively, more thanone dopant could be used evenly over the entire length of a single rodassuming the dopants did not interfere and reduce quantum efficiency.The length of the rods and excitation LED arrays could be increased toachieve higher flux out of collection optic 22. This is the primarydistinction and advantage of this technology over prior art comprised ofa thin planar luminescent material, as the out put can be increased byincreasing the length of the rod rather than increasing the powerdensity of the excitation source thereby resulting in output flux manymultiples of that which could be achieved by prior art. The output ofthe system of FIG. 4 could alternatively be directly coupled to anoptical fiber bundle without the need for re-imaging optics.

FIG. 5 represents alternative cross sectional areas for rods includingbut not limited to circular, square, rectangular, and multiple sidedpolygons such as a hexagon and octagon. Generally, even number of sidespolygons have better spatial mixing than those with an odd number ofsides although either could be used. Likewise, the optical concentratorthat would be index matched to one of the rod configurations could havea similar cross sectional shape. For example a rectangular or square CPCor taper could be used. A theta by theta CPC comprised of a tapercoupled to a CPC such as described by Welford and Winston (HighCollection Nonimaging Optics, W. T. Welford and R. Winston, AcademicPress, 1989) could be used.

FIG. 6 shows various configurations 100 of a combination of luminescentrod and output concentrators. For example the rods 102, 108, and 114,could be index matched to output couplers in the form of a taper 104,CPC 110, or combined theta by theta taper and CPC 116. In general theconcentrators would be made out of a material that is transparent and ofsimilar index of refraction and would be coupled by means of an indexmatching medium. Alternatively, the two components comprising a rod andconcentrator could be mated by heating the components and allowing themto melt together. Alternatively, the rod and concentrator could be madeout of the same material such as ceramic (phosphor particles sintered attemperatures on the order of 1800° Celsius and under pressure causingthe material to become transparent and substantially homogeneous) suchas Ce:YAG which could be doped in the region of the rod and not doped inthe region of the concentrator thereby eliminating the need for indexmatching.

FIG. 7 shows a plot of index of refraction of the concentrator versuscoupling efficiency for the case of Ce:YAG rod which has an index ofrefraction on the order of 1.82 for two rod geometries circular andsquare in cross section. The out-coupling efficiency into air (index ofrefraction 1) of 30% assumes that all the light emitted by the LED dieis absorbed within the rod and that one end of the rod is coated with amirror with reflectivity of 100%. Thus, the efficiency can be improvedby the order of 80% by index matching to a concentrator with an index ofrefraction approaching that of the rod. The data also assumes that theoutput face of the concentrator is anti-reflection coated to minimizelosses due to Fresnel reflections at the air/dielectric interface.

FIG. 8 shows empirical data for a white light source transmitted throughthe side of a Ce:YAG rod of 1 mm in thickness as well as guided down itslength of 50 mm. The Cerium doping was 0.15%. The data shows that forthe 1 mm path length more than 90% of the blue light was absorbed. Therod was not coated, so the maximum expected transmission would be on theorder of 84% due to Fresnel reflection which is observed at a wavelengthof about 400 nm where the Ce:YAG rod is substantially transparent. Thefact that the output is above the expected maximum transmission forwavelengths greater than 500 nm is due to the contribution from theluminescent light emitted by the absorbed blue light in the incidentwhite light. The broader absorption band shown in the 50 mm length isdue to the fact that Beer's Law is acting over 50 times the lengthexponentially. It is also apparent that the material does exhibit somedegree of self absorption for which some of the absorbed light emittedas phosphorescence is absorbed through the length. Thus for someapplications it may be important to limit the length of the rod tominimize absorption at the short end of the emitted spectrum and tominimize heating due to self absorption.

FIG. 9 shows the combined spectrum of the system of FIG. 1 with thethick black vertical lines representing the spectral region of thedichroic beam splitters. The current to the individual sources can beadjusted to result in a CRI greater than 90 at a CCT on the order of5700° Kelvin which is consistent with the values typical of short arcXenon lamps. The blue spectrum shown here is comprised of three blue LEDpeak wavelength centered around 445 nm, 457 nm and 470 nm. The red bandis comprised of the combination of LED center wavelengths peaked near630 nm and 650 nm. The effect of increasing the spectral widths in theblue and red spectral regions is primarily to increase the CRI.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. An illumination system comprising: a first lightsource, wherein the first light source includes a plurality of firstLight Emitting Diodes (LEDs), wherein the first light source emits afirst light beam of a first color; wherein said plurality of first LightEmitting Diodes (LEDs) of said first light source emit light of anexcitation color different than said first color, and wherein said firstlight source further comprises a luminescent rod comprising aluminescent material which absorbs said light of said excitation colorand, in response, emits light of said first color to generate said firstlight beam of said first color; a second light source, wherein thesecond light source includes a second Light Emitting Diode (LED),wherein the second light source emits a second light beam of a secondcolor different than the first color; a third light source, wherein thethird light source includes a third Light Emitting Diode (LED), whereinthe third light source emits a third light beam of a third colordifferent than the first color and second color; a plurality ofreflective optical components positioned so as to combine the firstlight beam of the first color, the second light beam of the secondcolor, the third light beam of the third color, on a main optical axisof the illumination system to generate a beam of white light; and anoutput system which focuses said beam of white light into a light guide;wherein the illumination system has the ability to individually adjustcurrent to each of the first light source, second light source, andthird light source.
 2. The illumination system of claim 1, wherein theability to individually adjust the current to each of the first lightsource, second light source, and third light source of the illuminationsystem enables the illumination system to control a correlated colortemperature (CCT) of the beam of white light.
 3. The illumination systemof claim 1, wherein the ability to individually adjust the current toeach of the first light source, second light source, and third lightsource of the illumination system enables the illumination system tocontrol a color rendering index (CRI) of the beam of white light.
 4. Theillumination system of claim 1, wherein the beam of white light has acolor rendering index (CRI) of at least
 85. 5. The illumination systemof claim 1, wherein the illumination system has the ability to modulatethe beam of white light.
 6. The illumination system of claim 1, whereinthe illumination system has the ability to modulate the light output insynchronization with a camera shutter.
 7. The illumination system ofclaim 1, in combination with a light guide positioned to receive saidbeam of white light from said output system, wherein the light guide isadapted to transport said beam of white light to a microscope or anendoscope.
 8. The illumination system of claim 1, in combination with alight guide and one of a microscope and an endoscope, wherein said lightguide is positioned to receive said beam of white light from said outputsystem and transport said beam of white light to said microscope orendoscope.
 9. The illumination system of claim 1, wherein the secondlight source emits a second light beam of a second color comprising aplurality of wavelengths of light of the second color.
 10. Theillumination system of claim 1, having a color rendering index (CRI)that is at least 90 and a correlated color temperature (CCT) that isapproximately 5700 Kelvin.
 11. An illumination system comprising: afirst light source, wherein the first light source includes a pluralityof first Light Emitting Diodes (LEDs), wherein the first light sourceemits a first light beam of a first color; wherein said plurality offirst Light Emitting Diodes (LEDs) of said first light source emit lightof an excitation color different than said first color, and wherein saidfirst light source further comprises a luminescent rod comprising aluminescent material which absorbs said light of said excitation colorand, in response, emits light of said first color to generate said firstlight beam of said first color; a second light source, wherein thesecond light source includes a second Light Emitting Diode (LED),wherein the second light source emits a second light beam of a secondcolor different than the first color; a third light source, wherein thethird light source includes a third Light Emitting Diode (LED), whereinthe third light source emits a third light beam of a third colordifferent than the first color and second color; a plurality ofreflective optical components positioned so as to combine the firstlight beam of the first color, the second light beam of the secondcolor, the third light beam of the third color, on a main optical axisof the illumination system to generate a beam of white light; and anoutput system which focuses said beam of white light into a light guide;wherein the illumination system has the ability to individually adjustcurrent to each of the first light source, second light source, andthird light source to control a correlated color temperature (CCT) and acolor rendering index (CRI) of the beam of white light.
 12. Theillumination system of claim 11, wherein the beam of white light has acolor rendering index (CRI) of at least
 85. 13. The illumination systemof claim 11, wherein the illumination system has the ability to modulatethe beam of white light.
 14. The illumination system of claim 11,wherein the illumination system has the ability to modulate the lightoutput in synchronization with a camera shutter.
 15. The illuminationsystem of claim 11, in combination with a light guide positioned toreceive said beam of white light from said output system, wherein thelight guide is adapted to transport said beam of white light to amicroscope or an endoscope.
 16. The illumination system of claim 11, incombination with a light guide and one of a microscope and an endoscope,wherein said light guides is positioned to receive said beam of whitelight from said output system and transport said beam of white light tosaid microscope or endoscope.
 17. The illumination system of claim 11,wherein the second light source emits a second light beam of a secondcolor comprising a plurality of wavelengths of light of the secondcolor.
 18. The illumination system of claim 17, wherein the third lightsource emits a third light beam of a third color comprising a pluralityof wavelengths of light of the third color.
 19. An illumination systemcomprising: a first light source, wherein the first light sourceincludes a plurality of first Light Emitting Diodes (LEDs), wherein thefirst light source emits a first light beam of a first color; whereinsaid plurality of first Light Emitting Diodes (LEDs) of said first lightsource emit light of an excitation color different than said firstcolor, and wherein said first light source further comprises aluminescent rod comprising a luminescent material which absorbs saidlight of said excitation color and, in response, emits light of saidfirst color to generate said first light beam of said first color; asecond light source, wherein the second light source includes a secondLight Emitting Diode (LED), wherein the second light source emits asecond light beam of a second color different than the first color; athird light source, wherein the third light source includes a thirdLight Emitting Diode (LED), wherein the third light source emits a thirdlight beam of a third color different than the first color and secondcolor; a plurality of reflective optical components positioned so as tocombine the first light beam of the first color, the second light beamof the second color, the third light beam of the third color, on a mainoptical axis of the illumination system to generate a beam of whitelight; and an output system which focuses said beam of white light intoa light guide; wherein the illumination system has the ability toindividually adjust each of the first light source, second light source,and third light source to control a correlated color temperature (CCT)and a color rendering index (CRI) of the beam of white light.
 20. Theillumination system of claim 19, wherein the beam of white light hassaid color rendering index (CRI) of at least 85.